You've got one of perhaps several bugs which change the permissions on a
registry key so that even an administrator doesn't have access, without
taking ownership.
At present, this foils the beta1 product. I expect that Microsoft will have
a better way to handle this in beta2.
If you are knowledgeable, you can find the key it is sticking on in the
registry and take ownership of it, which will allow the scan to continue and
complete.
Otherwise, I'd recommend some research on the bug involved, and using other
tools--perhaps Symantec will have an automated removal tool, or perhaps
McAfee's Stinger or an online scan from Trend Micro.